Investment casting, also referred to as the "lost wax" process, typically involves alternate applications of a ceramic coating composition and a stucco composition to an expendable pattern in order to provide a multi-layered shell mold. The pattern is usually mode of wax, plastic, or similar material which is melted out to leave a correspondingly shaped internal cavity into which molten metal is poured.
Unfortunately, there have been many attempts to control the surface finish and the amount of decarburization of steel investment castings. The problem of a metal-mold-atmosphere reaction at the time of pouring and initial stages of solidification of the molten metal has continued to cause an undesirable carbon-free zone adjacent the surface of the article as well as surface blemishes. The methods of minimizing this phenomenon have included casting in a vacuum, use of inert gas shrouding, the addition of reducing agents into the mold cavity prior to pouring, preheating the mold in a carbonaceous atmosphere prior to casting, etc. All of these production steps are costly, time-consuming or raise issues of safety to foundry personnel such as by producing noxious vapors.
U.S. Pat. No. 3,184,813 issued to P. J. O'Shea on May 25, 1965 and U.S. Pat. No. 3,296,666 issued to N. G. Lirones on Jan. 10, 1967 are representative of the large number of ceramic dip coat compositions used in building up multi-layered shell molds. Frequently, the compositions of the shell mold layers are tailored for the specific metal.
In the past, for example, graphite has been added to the usual coating composition of a ceramic powder and a binder in order to improve surface finish and to minimize the amount of decarburization of steel articles. But while the use of a relatively uniform amount of graphite throughout the full cross section of the shell mold wall has resulted in some improvement in the quality of the castings, surface irregularities and localized carburization have been observed because of the undesirable contact of the molten metal directly with the graphite particles. Moreover, the strength of the individually applied layers is reduced by graphite addition and the shell mold is more costly than desired.
The present invention is directed to overcoming one or more of the problems as set forth above.